The ability to locate the position of a radio transmitter in a radio communication network, for example locating a mobile communication unit operating in a radio communication system such as GSM (Global System for Mobile Communication), provides many well-known advantages. Exemplary uses of position locating capability include security applications, emergency response applications, and travel guidance applications.
Conventional position locating techniques require the capability of making timing measurements on selected radio signals. For example, a plurality of measurement nodes located throughout the communication network may make time of arrival (TOA) measurements on a radio signal from a selected mobile communication unit. With knowledge of the time of arrival of a given radio signal at various measurement nodes at known locations in the network, the location of the mobile communication unit can be estimated using conventional time difference of arrival (TDOA) techniques.
In order to accurately determine the time of arrival of the radio signal, conventional measurement nodes are often provided with GPS (Global Positioning System) receivers to receive highly accurate clock signals from satellites orbiting the earth. Thus, the various measurement nodes ideally utilize the same absolute time base (GPS time) to make their time of arrival measurements.
One problem with the aforementioned use of GPS time in measurement nodes is that, for example in urban environments, sometimes only one or a few satellites will be visible to the GPS receiver in a given measurement node. Moreover, the one or few satellites visible to a first measurement node may be different than the one or few satellites visible to a second measurement node. If the position of the GPS receiver (i.e., the position of the measurement node) is known, it is sufficient to see only one satellite to obtain the absolute GPS time reference.
However, the accuracy of the absolute time reference obtained in this manner is typically low, mainly due to the conventional Selective Availability (SA) error which is intentionally induced in each satellite's GPS clock signal to degrade the accuracy of the signal, for global security reasons. Although the SA error has a zero mean over a long period (many hours) of time, it is random and, therefore, unpredictable on a short time basis. The SA error of a given satellite is independent of the SA errors of other satellites, but is correlated between different geographical locations. That is, the SA error in the GPS timing signal received from a given satellite will be approximately equal at two different measurement nodes if the geographical distance between those two measurement nodes is sufficiently small.
Due to the SA error, the accuracy of a GPS timing reference obtained from only a single satellite is poor, having a standard deviation of, for example, 100-200 ns. If a first measurement node sees only a first satellite and a second measurement node sees only a second satellite which is different from the first satellite, then the difference between the absolute time references respectively obtained by the two measurement nodes can range from, for example, 2.times.100 ns to 2.times.200 ns because the SA error is uncorrelated between different satellites. On the other hand, if the two measurement nodes could obtain their absolute time references from the same satellite, then the difference between the absolute time references of the two measurement nodes would be on the order of 2.times.30 ns.
The potential of relatively large differences between the absolute time references of the measurement nodes makes it difficult to compare the time of arrival measurements from the measurement nodes to obtain time difference of arrival information that is conventionally used to locate the radio transmitter. In other words, if the measurement nodes do not have the same time base, then this adversely impacts the ability to estimate the location of the transmitter using conventional time difference of arrival (TDOA) techniques.
Differences in absolute time base cause similar problems when stationary reference mobile units equipped with GPS receivers are used to measure the radio transmission timing of associated fixed-site transceivers in a radio communication network. When the measured transmission timings are compared in order to determine a real-time difference between the fixed-site transceivers, the difference between the respective absolute time bases of the reference mobile units adversely affects the calculation of the real-time difference.
It is therefore desirable to provide position locating techniques which avoid the aforementioned disadvantages of differing GPS time bases in respective measurement nodes.
The present invention provides a position locating technique wherein information indicative of satellite timing differences between the time references provided by different satellites is used to account for the satellite timing differences when comparing radio signal timing measurements provided by measurement nodes.